Although there is compelling evidence that the beta-amyloid peptide (Abeta) is centrally involved in Alzheimer's disease pathology, the mechanisms of Abeta toxicity and its specific targets remain unresolved. This project is a continuation of our studies using the intensely studied nematode worm Caenorhabditis elegans as a model to investigate the basis of (human) Abeta toxicity. We have previously shown that transgenic worms engineered to express human Abeta 1-42, replicate some aspects of Alzheimer's disease pathology. By a combination of molecular and genetic approaches, we have now identified a set of conserved genes that modulate Abeta toxicity in these transgenic worm models. In addition, analysis of transgenic worms expressing single amino acid variants has led to the identification of an Abeta variant that is substantially non-toxic in vivo. The goal of this proposal is to synthesize these findings to establish: 1) the identity of the toxic Abeta species, 2) the cellular pathways that influence the formation of the toxic species, and 3) the specific cellular targets of Abeta. The Specific Aims of this proposal are to: 1) perform an in vivo structure/function analysis of Abeta toxicity by constructing and characterizing transgenic worms expressing variant Abeta peptides. These studies will directly test the "toxic Abeta oligomer" model, 2) determine the molecular mechanisms by which evolutionarily conserved modifier genes alter Abeta toxicity, and 3) validate the proposed mechanisms for both Abeta toxicity and the protective genes we have identified using mammalian cell culture and primary neuronal cultures. Our proposed studies have direct relevance for the diagnosis and treatment of Alzheimer's disease. The identity of the key toxic form(s) of Abeta may be critical for designing drugs that prevent its formation or toxic activity. Characterization of genes that affect the toxicity of Abeta may be important for both risk assessment and the development of effective interventions for Alzheimer's and other neurodegenerative diseases.